Final answer:
Liquid water is denser than ice because, at 4°C, water molecules are packed closely, whereas ice has a crystalline lattice structure with more open space due to hydrogen bonding. This causes ice to float on water. Water's expansion upon freezing has critical implications for marine life survivability and practical concerns like the use of antifreeze.
Step-by-step explanation:
The unique properties of water as it transitions to ice involve density changes related to its molecular structure. Liquid water achieves its highest density at around 4°C due to the molecules being packed closely together. As water cools further and freezes, the hydrogen bonds organize the water molecules into a crystalline lattice that is less efficient, creating a more open structure. The resulting ice has a less dense structure than the liquid water, causing it to float.
Hydrogen bonding plays a crucial role in this process. In liquid water, hydrogen bonds are constantly breaking and reforming, allowing the water molecules to move past one another closely and compactly. In contrast, when water freezes, the hydrogen bonds hold the molecules in a rigid structure where they are spaced further apart compared to liquid water. This gives ice a lower density than the water it forms from, which is why ice floats on water and why bodies of water freeze from the top down, creating a protective layer that insulates the rest of the water and allows aquatic life to survive in cold temperatures.
This behavior of water is unique among most substances, as typically solids are denser than their liquid forms. The expansion of water when freezing also has important practical implications, such as the need for antifreeze in engines and the risk of bursting pipes during freezing weather.